In recent years, as information has become ubiquitous, it has become necessary for mobile information terminals to also have processing performance matching that of personal computers. In accompaniment with this, it is also demanded that image display devices have high-resolution and high picture quality, and it is desirable for such image display devices to be thin, be lightweight, be visible from wide angles, and have low power consumption.
In order to respond to such requirements, display devices (displays) have been developed where thin film active elements (thin film transistors, also referred to as TFTs) are formed on a glass substrate, with optoelectronic elements then being formed on top.
In the main, a substrate forming active elements is such that patterning and interconnects formed using metal are formed after forming a semiconductor film of amorphous silicon or polysilicon etc. Due to differences in the electrical characteristics of the active elements, the former requires ICs (Integrated Circuits) for drive use, and the latter is capable of forming circuits for drive use on the substrate.
With liquid crystal displays (Liquid Crystal Displays or simply LCDs) currently widely in use, the former amorphous crystal type is widespread for large-type screens, while the latter polysilicon type is common for medium and small-type screens.
Of self-luminous type screens, polysilicon type displays are the only electroluminescent (organic EL) displays characterized by being thin, light-weight and having a wide angle of visibility that are mass-produced.
Typically, organic EL elements are used in combination with TFTs and utilize this voltage/current control operation so that current is controlled. The current/voltage control operation referred to here refers to the operation of applying a voltage to a TFT gate terminal so as to control current between the source and drain. As a result of doing this it is possible to adjust the intensity of emitted light from the organic EL element and to display with the desired gradation.
However, because this configuration is adopted, the TFT characteristic is extremely sensitive to the influence of the intensity of light emitted by the organic EL element. In particular, for polysilicon TFTs formed using low-temperature processes referred to as low-temperature polysilicon, it can be confirmed that comparatively large differences in electrical characteristics occur between neighboring pixels. This is a major cause of deterioration of the display quality of organic EL displays, in particular, the uniformity of displaying within a screen.
Related art for improving this is disclosed in patent document 1. In patent document 1, the polysilicon TFTs driving the organic EL element are driven so as to be in one of two states, either lit-up, or extinguished (digital driving). This suppresses variations in the characteristics, and this enables gradation as a result of controlling this illumination period. Namely, in order to control the illumination period of the organic EL, a plurality of drive circuits capable of a plurality of scans are added.
In Japanese Patent Laid-open Publication No. 2002-29709 the number of polysilicon TFT circuits is increased because of, for example, adding a plurality of driver circuits constituted by polysilicon TFTs in order to achieve digital driving. The number of polysilicon TFT circuits is therefore increased, and the circuit failure rate therefore increases accordingly. In particular, a high-definition display panel will have a very large number of pixels and drive circuits, which will cause yield to fall and costs to increase.
It is therefore advantageous for the present invention to implement a high-quality organic EL display for which the number of circuits for digital driving is kept small and display uniformity is high.